1. Field of the Invention
The present invention relates to an optical disc apparatus and a defocus value setting method thereof, capable of recording a signal on an optical disc by laser light irradiated by an optical pickup unit and reproducing the signal recorded on the optical disc by the laser light.
2. Description of the Related Art
Laser light irradiated by an optical pickup unit (OPU) of an optical disc apparatus is focused on a signal face of an optical disc. Laser is an abbreviation of “light amplification by stimulated emission of radiation”. Various kinds of technology of focusing the laser light on the signal face of the optical disc or so-called focus control technology are available. Generally, focus control is performed using a signal obtained from a photodetector incorporated in the optical pickup unit.
The focus means a focal point or a focal point of lens. Focusing means brining into focus or being brought into focus. A defocus adjustment as used in the present specification means, for example, an adjustment work of bringing into focus something that is out of focus.
Description will be made of the focus control found in publications. A focus control device, and a focus pull-in method, is known that performs a focus pull-in operation in a short time and securely in an optical disc whose reflection ratio varies depending on the state or a phase-change optical disc whose reflection ratio varies depending on heat of irradiated light (see, e.g., Japanese Patent Application Laid-Open Publication No. 2002-342948 (pp. 1 and 3 to 5, FIG. 1-5)).
The focus control is performed by a circuit called a focus servo circuit and displacement movement of an objective lens is performed, for example, with the position of movement center of the objective lens used as a reference. Servo means a mechanism, etc., of measuring a state of an object to be controlled, comparing thus measured value with a predetermined reference value, and automatically controlling for correction.
For example, there is the optical pickup unit equipped with a system that is capable of applying an arbitrary offset by a focus mechanism and adjusts a jitter at the time of reading out data from a disc by a focus offset or so-called defocus. The jitter means a subtle vibration or distortion of the signal.
For example, the optical pickup unit is known in which the position of the movement center of the objective lens is adjusted by detecting a value of the jitter contained in the signal such as a reproduced signal (see, e.g., Japanese Patent Application Laid-Open Publication No. 1995-262584 (pp. 1 and 2, FIG. 1-6)).
Usually, in the optical pickup unit, an F bias value of the focus or so-called defocus value is varied and the defocus value is set at an optimum jitter value.
For example, an optical disc reading unit is known that, by detecting the correlation between the focus bias volume of displacing a reference position of the focus of the pickup and the jitter value of the data read out by the pickup, can obtain the focus bias volume corresponding to the optical disc (see, e.g., Japanese Patent Application Laid-Open Publication No. 1998-228652 (pp. 1 and 2, FIG. 1-8)).
The focus control in the optical disc apparatus is performed utilizing a focus error signal obtained by the photodetector. When the objective lens is displaced by the focus control substantially along the direction orthogonal to the face of the optical disc, the focus error signal called an S-shaped curve is obtained, for example, as described in the above Japanese Patent Application Laid-Open Publication No. 2002-342948. Then, when the level of the focus error signal comes within a focusing controllable range with its center at a point called zero cross of the S-shaped curve, the focus servo operation is performed and the focus control is executed. With the focus servo operation being performed, the operation of focusing the laser light irradiated by a laser diode on the signal face of the optical disc is performed.
Conventional optical pickup units had the problem that due to the difference between individual pickup units themselves and the difference between individual optical discs, a focused focal point controlled by the focus point and the focus servo operation was not performed in optimum condition.
To solve the above problem, for example, the optical disc reading unit was proposed that is shown in the above Japanese Patent Application Laid-Open Publication No. 1998-228652. However, in this optical disc reading unit, it is feared, for example, that since the focus bias volume is set by arithmetic processing, a long time is required for setting processing.
When a method of setting the focus bias volume based on the minimum value of the jitter value is employed, the following problem is feared. For example, some of the optical discs used have very slight change of the jitter value. The bias volume set in the case of such optical discs sometimes became the focus bias volume at the edge of the detecting range. When the focus bias volume was set as above, there was a problem that the focus servo operation became unstabilized.
Detailed description will now be given. With respect to some optical discs, when the defocus value was adjusted by the optical pickup unit, the jitter value corresponding to the defocus value hardly changed and with respect to other optical discs, the jitter value became reading-stable jitter value even if the defocus value was set at 0. When the defocus value is set to meet the optimum jitter with respect to such optical discs, such defocus value makes the servo unstable and causes a loss of focus or so-called F loss, etc. to take place.
The loss of focus means, for example, that a focal point of the laser light irradiated from the optical pickup unit is caused to deviate relative to pits of the disc being tracked and thereby the data stored on the disc becomes unreadable. A pit means a hole or depression.
For example, when the defocus adjustment is made based on the detected jitter, the F loss sometimes happened in the optical pickup units that had small jitter change. It was possible that at the time of the defocus adjustment, the optical pickup unit that had small jitter change was set at the defocus value away from the center value corresponding to the movement center of the objective lens. In the optical pickup unit with such setting made, the F loss sometimes happened.